Structural First-order Transition Research Articles

Superconductivity versus structural phase transition in the closely relatedBi2Rh3.5S2andBi2Rh3S2

Single crystals of Bi$_{2} $Rh$ _{3}$S$ _{2}$ and Bi$_{2} $Rh$ _{3.5}$S$ _{2}$ were synthesized by solution growth and the crystal structures, thermodynamic and transport properties of both compounds were studied. In the case of Bi$_{2} $Rh$ _{3}$S$ _{2}$, a structural first-order transition at around 165 K is identified by single crystal diffraction experiments, with clear signatures visible in resistivity, magnetization and specific heat data. No superconducting transition for Bi$_{2} $Rh$ _{3}$S$ _{2}$ was observed down to 0.5 K. In contrast, no structural phase transition at high temperature was observed for Bi$_{2} $Rh$ _{3.5}$S$ _{2}$, however bulk superconductivity with a critical temperature, $T_{c}\approx$ 1.7 K was observed. The Sommerfeld coefficient $\gamma$\ and the Debye temperature($\varTheta_{\textrm{D}}$) were found to be 9.4 mJ mol$^{-1}$ K$^{-2}$ and 209 K respectively for Bi$_{2} $Rh$ _{3}$S$ _{2}$, and 22 mJ mol$^{-1}$ K$^{-2}$ and 196 K respectively for Bi$_{2} $Rh$ _{3.5}$S$ _{2}$. Study of the specific heat in the superconducting state of Bi$_{2} $Rh$ _{3.5}$S$ _{2}$ suggests that Bi$_{2} $Rh$ _{3.5}$S$ _{2}$ is a weakly coupled, BCS superconductor.

Experimental access to elastic and thermodynamic properties of RbMnFe(CN)6

We use spectroscopic ellipsometry to study the elastic and thermodynamic properties of the structural first-order transition of the cooperative Jahn–Teller solid, RbMn[Fe(CN)6]. While the analysis of the thermal dependence of the dielectric constant revealed a remarkable energy shift in the metal-to-ligand charge transfer band, that of the refractive index allowed to evaluate the volumetric thermal expansion coefficient in the two phases through the Gladstone–Dale relation. The access to the elastic properties of the solid is obtained with the reflectivity under pressure measurements, from which we estimated the bulk modulus values in the low- and high-temperature phases as BLT=30(±3) GPa and BHT=23(±2) GPa, respectively. Assuming the system is isotropic, the corresponding Debye temperature values have been found to be θDLT(SE)≃360(±25) K and θDHT(SE)≃290(±20) K, in good agreement with those derived from independent Mössbauer spectrometry investigations which led to θDLT(Moss)≃332(±7) K and θDHT(Moss)≃280(±11) K. Moreover, we have been able to extract from the SE data, the entropy change at the transition, ΔS≈64 J K−1 mol−1, which has been also found in good agreement with the value, ΔS≈60±5 J K−1 mol−1, derived from calorimetric experiments performed in this work.

Magnetic and Structural Phase Transitions in Monochalcogenides of Uranium

The theory of simultaneous magnetic and structural first-order transitions in crystals with strong cubic magnetic anisotropy is developed on the basis of a combination of the magnetic modified 8-state Potts model [1] and theoretical models of structural phase transitions in cubic crystals [2]. The magnetic heat capacity and specific volume, as well as the temperature dependence of the rhombohedron angle, are calculated in a wide temperature range. The calculation of the temperature evolution of high-temperature diffuse magnetic neutron scattering was performed. It was demonstrated that upon passing the Curie point the spectra characteristic of the diffuse scattering are transformed to the magnetic Bragg peaks.

Magnetic and structural transitions in crystals with a structure of the NaCl type

A model of simultaneous magnetic and structural first-order transitions in antiferromagnets with a strong cubic magnetic anisotropy has been constructed on the basis of a synthesis of magnetic modified 6-state and 8-state Potts models and the theoretical model of structural phase transitions in cubic crystals. A revised scheme has been suggested for the derivation of possible magnetic structures in the fcc lattice with allowance for competing interactions between the nearest and next-nearest neighbors. A calculation of the temperature evolution of high-temperature diffuse magnetic scattering of neutrons has been carried out to show that the mechanism of a magnetic transition at the Neel point is caused by the transformation of diffuse magnetic scattering into magnetic Bragg peaks.

10.1007/s11508-008-3001-5

A model of simultaneous magnetic and structural first-order transitions in crystals with strong cubic magnetic anisotropy has been constructed on the basis of a combination of the magnetic modified 8-state Potts model [1] and theoretical models of structural phase transitions in cubic crystals [2]. The magnetic heat capacity and specific volume, as well as the temperature dependence of the rhombohedron angle, have been calculated in a wide temperature range. The calculation of the temperature evolution of high-temperature diffuse magnetic neutron scattering has been performed to show that upon the passage through the Curie point it is converted into magnetic Bragg peaks.

Optical absorption and phase transitions in Cu-III-VI 2 compound semiconductors at high pressure

The optical absorption edge of CuInTe 2, CuGaS 2 and CuGaSe 2 single crystals was measured as a function of hydrostatic pressure up to 20 GPa. In all cases the energy gap increases with pressure. From the data the pressure dependence of the energy gap ( dE g dP ) was determined as 22 meV GPa −1 for CuInTe 2, 40 meV GPa −1 for CuGaS 2 and 50 meV GPa −1 for CuGaSe 2. From changes in the light transmission characteristic of the samples under pressure, structural first-order transitions were observed at 2.8 GPa for CuInTe 2, 16.5 GPa for CuGaS 2 and 13.6 GPa for CuGaSe 2. Also, a new metastable semiconducting phase was formed in pressure-quenched CuGaS 2 samples.

XY-to-Gaussian crossover for the Potts-model transition near the bicritical point of stressed SrTiO3

The trigonal-to-pseudotetragonal structural first-order transition in [111]-stressed SrTi${\mathrm{O}}_{3}$, realizing the three-state Potts model, is studied in the close vicinity of the bicritical point (BCP). The exponents describing the effects of fluctuations on this transition are shown to cross over from $\mathrm{XY}$- to Gaussian-model values as the BCP is approached. The first-order line is shown to approach the BCP tangentially to the temperature axis.